1. Field of the Invention
The present invention relates to a positive electrode active substance, a positive electrode, and a non-aqueous electrolyte secondary battery. More particularly, the present invention relates to a positive electrode active substance that gives a non-aqueous electrolyte secondary battery excellent in cycle characteristics as well as to a positive electrode and a non-aqueous electrolyte secondary battery using the same.
2. Description of the Related Art
As a non-aqueous electrolyte secondary battery, a lithium secondary battery is put into practical use and is widely prevalent. Further, in recent years, a lithium secondary battery is attracting people's attention not only as a small one for a portable electronic apparatus but also as a large-capacity device for being mounted on a vehicle or for electric power storage. For this reason, there is an enhanced demand for safety, cost reduction, long lifetime, and the like.
The lithium secondary battery has a positive electrode, a negative electrode, an electrolytic solution, a separator, and an outer cladding material as principal constituent elements. Also, the above positive electrode is constituted of a positive electrode active substance, an electroconductive material, a collector, and a binder (binding agent).
Generally, as the positive electrode active substance, a layered transition metal oxide such as represented by LiCoO2 is used. However, the layered transition metal oxide is liable to provoke oxygen elimination in a fully charged state at a comparatively low temperature around 150° C., and this oxygen elimination can provoke thermal bursting reaction of the battery. Therefore, when the battery having such the positive electrode active substance is used in the portable electronic apparatus, there is a fear that an accident such as heat generation and fire catching of the battery may occur.
For this reason, lithium iron phosphate (LiFePO4) having a stable structure that does not release oxygen at an abnormal time and having an olivine structure less expensive than LiCoO2 is now expected.
It is known that LiFePO4 has a volume change ratio (See the following formula) as large as about 7% between at the time of Li intercalation and at the time of Li deintercalation, and generates capacity deterioration by repetition of charging and discharging cycles. The reason why the capacity deterioration is generated is as follows. Namely, volume change caused by repetition of charging and discharging cycles provokes destruction of a particulate positive electrode active substance made of LiFePO4, disconnection of the electroconductive path, and the like. By destruction, disconnection, and the like, a rise in an internal resistance within the positive electrode and inactive parts are generated, thereby causing capacity deterioration (decrease).volume change ratio=(A−B)×100/A where A is a unit lattice volume before lithium deintercalation and B is a unit lattice volume after lithium deintercalation.
Also, it is known that, at a high temperature, a reaction product produced at the interface between the non-aqueous electrolyte and the positive electrode deteriorates the capacitance.
Various methods of solving the aforementioned capacity deterioration are proposed.
For example, Japanese Patent Application Publication No. 2005-340056 attempts to restrain capacity deterioration by putting Al2O3 that does not contribute to charging and discharging into the positive electrode.
Also, Japanese Patent Application Publication No. 2008-166207 attempts to restrain capacity deterioration by putting an inorganic substance that does not contribute to charging and discharging into the positive electrode so as to raise the dispersibility of the positive electrode active substance.